This invention relates to an arrangement for compensating for detrimental magnetic horizontal fields at the plus or upstream end of pots arranged longitudinally in plants for producing aluminium by electrolytic reduction in a molten bath.
In Norwegian Pat. No. 122,680 there is described an arrangement for compensating for the detrimental magnetic influence on a row of pots from another pot row in plants for producing metal, for instance aluminium, by electrolytic reduction. The form of compensation with which the present invention is concerned, is of particular interest in such plants where there is provided for compensation of the detrimental influence between pot rows in the electrolyzing plant. However, the present invention is not limited to use in such plants, but can also be utilized in plants in which there is no significant mutual magnetic influence between the pot rows, for example in pot rooms with a large space between the pot rows.
In pots for the electrolysis of a molten bath the electrolysis current will flow substantially in a vertical direction between the anode and the cathode. In association with the current flowing in riser conductors, the anode conductor and bus bars there is generated a horizontal magnetic field. This field sets up movements in the electrolyte or molten bath and in the metal, and causes a curvature of the metal surface. These movements and this curvature are detrimental and according to experience result in an instable pot operation and reduced current yield.
Various precautions have previously been known for reducing or eliminating the detrimental effects of undesired magnetic fields in electrolytic cells or pots, these also including horizontal magnetic fields of the kind which are to be compensated for by the present invention. One example may be found in British Pat. No. 794,421, which, however, attempts to limit the detrimental effects by modifying the distribution of the electrolysis current in the metal sump. In the present invention, however, the attention is primarily directed to the undesired magnetic field components, in particular for the purpose of obtaining full symmetry and a better field distribution of the horizontal field component laterally of the pot. This method also influences the movements in the electrolytic bath, and these have as much significance as the movements in the metal sump.
It is emphasized that the present invention is exclusively directed to plants having the pots arranged longitudinally in the pot rows, which within the smelting industry is regarded as an arrangement being fundamentally different from an arrangement with laterally oriented pots. Norwegian Pat. No. 124,318 is directed to an arrangement of laterally oriented electrolytic cells, and to the provision of bus bars underneath these lateral pots. This known arrangement in connection with lateral pots has not, however, been of any assistance to experts occupied with the problems of irregularities in magnetic horizontal fields in longitudinally oriented pots for producing metal, for example aluminium, by electrolytic melting. The current and field relationships in electrolytic cells or pots are so complicated that in practice it is about impossible to transfer experience or solutions with respect to magnetic field compensation from laterally oriented pots to longitudinally oriented pots or vice versa. This also applies to a high degree to the problems of detrimental magnetic horizontal fields.
In plants with longitudinally oriented pots the current is usually conducted to each pot with a main proportion to one end thereof, designated the plus or upstream end, whereas a smaller proportion is conducted to the other end, namely the minus or downstream end. According to Norwegian Pat. No. 122,680 all current to the minus end of the pot is conducted along that side which faces the adjacent pot row. In order to obtain a bus bar system which is as economical as possible, the compensation conductor is arranged as close to the pot as possible, so that there is needed no more than one example approximately 30 kA for obtaining a full compensation of a pot for 150 kA. Such a current distribution gives a favourable vertical field, but the horizontal field will be too strong and will have an unfavourable distribution at the plus end of the pot.